Just a few days ago, Climate Central asked Mark Serreze, director of the National Snow and Ice Data Center (NSIDC) in Boulder Colorado, when sea ice in the Arctic Ocean was likely to reach its annual minimum. “Typically,” he said, “it happens around September 15, so the minimum will most likely come in about a week, but it can vary. Last time we checked [on Thursday] it was still going down.” By the time the resulting story was published on Saturday, Sept. 11, it turns out, it was going up.
Having waited a few more days until they were (almost) sure, NSIDC has just announced that last Thursday indeed appears* to have been the day of the minimum: at that point, the ice had receded to cover 4.76 million square kilometers of ocean — the third-lowest reading since satellite observations (the only reliable way of continuously measuring ice extent) began in 1979. Only 2007 and 2008 had less ice coverage, and this year, for the third summer in a row, the Northwest Passage and the Northeast Passage, along Russia’s northern coast, were open simultaneously. Walruses were feeling the heat as well, with a mass haulout event taking place in Alaska.
This graph shows how the ice has fared in 2010, compared with the record minimum in 2007 and the 1979-2000 average.
Credit: National Snow and Ice Data Center.
Climate scientists are generally wary of declaring long-term trends based on a year or two's worth of data, and Serreze’s statement in 2008 that Arctic ice was now in a “death spiral” that could leave the region ice-free in late summer within a couple of decades or sooner was seen by some as a bit extreme — especially given that the ice had rebounded somewhat that very year. In an interview with Grist.org in 2009, Serreze explained:
“As for the 'great recovery' of ice extent in 2008 heard in some circles, it was a recovery from lowest (2007) to second lowest (2008). I find little room for optimism here… Spring is increasingly dominated by thin, first-year ice prone to melting out in summer; As the thin ice now starts to melt out earlier in summer, the albedo feedback is stronger meaning even more summer melt; Arctic is warming in all seasons, meaning that recovery through a series of cold years is becoming less and less likely. Take these three together, and you are probably looking at ice-fee summers by 2030. I’d call that a death spiral.”
He used the term again just a few days ago. Others have pegged the probable ice-free date closer to 2040. But the precise date is less important than the fact that vanishing Arctic sea ice is one of the hallmarksof human-triggered climate change. And while a couple of years of unusual meltback wouldn’t prove all that much, it’s now more than just a couple.
* The NSIDC says this about why it's not ready to say more definitively that we've passed the actual minimum: “Because of the variability of sea ice at this time of year, the National Snow and Ice Data Center determines the minimum using a five-day running mean value. We have now seen four days of gains in extent. It is still possible that ice extent could fall slightly, because of either further melting or a contraction in the area of the pack due to the motion of the ice. For example, in 2005, the time series began to level out in early September, prompting speculation that we had reached the minimum. However, the sea ice contracted later in the season, again reducing sea ice extent and causing a further drop in the absolute minimum. When all the data for September are in, we will confirm the minimum ice extent for the season.”